This invention is directed to a process and element capable of forming a useful redox couple in response to actinic radiation in excess of 300 nanometers in wavelength. More specifically, this invention is directed to a photographic process and element capable of selectively generating a useful redox couple through the interaction of a cobalt(III)complex and a photoreductant. The present invention is further concerned with a photographic element and process capable of forming a photographic image in either a photographic element or layer containing the redox couple or in a separate, contiguous photographic element or layer.
Classically, photograhic elements have incorporated silver halide as a radiation-sensitive material. Upon exposure and processing the silver is reduced to its metallic form to produce an image. Processing, with its successive aqueous baths, has become increasingly objectionable to users desiring more immediate availability of a photographic image. Despite the processing required, silver halide photography has remained popular, since it offers a number of distinct advantages. For example, although silver halide is itself photoresponsive only to blue and shorter wavelength radiation, spectral sensitizers have been found which, without directly chemically interacting, are capable of transferring longer wavelength radiation energy to silver halide to render it panchromatic. Additionally, silver halide photography is attractive because of its comparatively high speed. Frequently, silver halide is referred to as exhibiting internal amplification--i.e., the number of silver atoms reduced in imaging is a large multiple of the number of photons received.
A variety of nonsilver photographic systems have been considered by those skilled in the art. Typically these systems have been chosen to minimize photographic processing and to provide useable photographic images with less delay than in silver halide photography. Characteristically, these systems require at least one processing step to either print or fix the photographic image. For example, ammonia or heat processing has been widely used in diazo imaging systems. While advantageously simple in terms of processing, these systems have, nevertheless, exhibited significant disadvantages. For example, many nonsilver systems are suitable for producing only negative images (or only positive images). Further, these systems have been quite slow, since they have generally lacked the internal amplification capability of silver halide. Many systems have also suffered from diminishing image-background contrast with the passage of time.
The use of cobalt(III)complex compounds in photographic elements is generally known in the art. For example, Shepard et al U.S. Pat. No. 3,152,903 teaches imaging through the use of an oxidation-reduction reaction system that requires a photocatalyst. The solid reducing agent is taught to be any one of a number of hydroxy aromatic compounds, including dihydrophenols, such as hydroquinone. The oxidant is taught to be chosen from a variety of metals, such as silver, mercury, lead, gold, manganese, nickel, tin, chromium, platinum, and copper. Shepard et al does not specifically teach the use of cobalt(III)complexes as oxidants. Instead, Shepard et al teaches that photochromic complexes, such as cobalt ammines, can be employed as photocatalysts to promote the oxidation-reduction reaction.
Cobalt(III)complexes are known to be directly responsive to electromagnetic radiation when suspended in solution. While most cobalt(III)complexes are preferentially responsive to ultraviolet radiation below about 300 nanometers, a number of cobalt(III)complexes have been observed in solution to be responsive to electromagnetic radiation ranging well into the visible spectrum. Unfortunately, these same complexes when incorporated into photographic elements lose or are diminished in their ability to respond directly to longer wavelength radiation. For example, Hickman et al in U.S. Pat. No. 1,897,843 teaches mixing thio-acetamide with hexamino cobaltic chloride to form a light-sensitive complex capable of interacting with lead acetate to produce a lead sulfide image. Hickman et al U.S. Pat. No. 1,962,307 teaches mixing hexammine cobaltic chloride and citric acid to form a light-sensitive complex capable of bleaching a lead sulfide image. Weyde in U.S. Pat. No. 2,084,420 teaches producing a latent image by exposing Co(NH.sub.3).sub.2 (NO.sub.2).sub.4 NH.sub.4 to light or an electrical current. A visible image can be formed by subsequent development with ammonium sulfide. In each of the above patents there is no photoreductant present.
Borden in U.S. Pat. No. 3,567,453, issued Mar. 2, 1971, and in his article "Review of Light-Sensitive Tetraarylborates", Photographic Science and Engineering, Volume 16, No. 4, July-August 1972, discloses that aryl borate salts incorporating a wide variety of cations can be altered in solvent solubility upon exposure to actinic radiation. Borden demonstrates the general utility of aryl borate salts as radiation-sensitive compounds useful in forming differentially developable coatings, as is typical of lithography, by evaluating some 400 different cations ranging from organic cations, such as diazonium, acridinium and pyridinium salts, to inorganic cations, such as cobalt hexammine. Borden discloses that the aryl borate salts can be spectrally sensitized with a variety of sensitizers, including quinones. In its unsensitized form the cobalt hexammine tetraphenyl borate of Borden is reported to be light sensitive in the range of from 290 to 430 nanometers. Borden notes in his report that hexammino cobalt chloride, although bright orange and therefore absorptive in the visible spectrum, is not useful in the lithographic system discussed in his article. Thus, Borden relies upon the light-sensitive aryl borate anionic moiety to provide radiation sensitivity.
In patent applications Ser. Nos. 384,858, now U.S. Pat. No. 3,887,372; 384,859, now U.S. Pat. No. 3,887,374; 384,860, now U.S. Pat. No. 3,880,659 and 384,861, now abandoned; all filed Aug. 2, 1973, it is taught to reduce tetrazolium salts and triazolium salts to formazan and azo-amine dyes, respectively, employing in the presence of labile hydrogen atoms a photoreductant which is capable of forming a reducing agent precursor upon exposure to actinic radiation. The reducing agent precursor is converted to a reducing agent by a base, such as ammonia.
Imaging systems have been developed which rely upon the oxidation of leuco dyes or upon the unblocking of a blocked color coupler or dye to form an image. Representative examples can be found in U.S. Pat. No. 3,615,565, British Pat. No. 975,457 and Research Disclosure, vol. 126, October 1974, Publication No. 12617, Para. III(E)2). These do not however achieve amplification by reason of the oxidation or the unblocking mechanisms.